Gears are probably the most efficient method of mechanical power transmission we have today, and much of the reason for that is the shape that gear teeth are ground to.

Unlike the triangular or rectangular teeth you see on comical depictions of gear trains, gear teeth are actually ground to a shape called an "involute".

If you have a string wound around a cylinder, and you keep that string tight as you unwind it from the cylinder, the shape of the curve the end of that string makes is called an "involute". It's a shape similar to a spiral.

The unique advantage of involute gear teeth is that when two involute gear teeth meet, they actually ROLL over each other. That is, the point of contact between the two teeth is a ROLLING contact, not a sliding contact.

And it's this rolling contact made possible by the involute shape of the gear teeth that results in negligible loss of mechanical power to friction, and therefore the excellent mechanical efficiency of gear trains.

If gear teeth slid across each others surface, as is depicted in comical gear trains with rectangular teeth or the "peg" shaped gear teeth you might see on a TV show like "Gilligan's Island", then much of the mechanical power would be lost to friction.

Watch the gear animation at the top of this web page:
Involute gear - Wikipedia, the free encyclopedia

The black dot is the point of contact between the two gear teeth. Note that that dot moves with respect to both teeth as the teeth roll over each other, thereby transmitting force (and hence power) between the gear teeth with negligible friction losses.

So, gear trains are a very efficient way to transport power for much the same reason that passenger and cargo trains are a very efficient way to transport people and goods... ... both benefit from negligible friction losses as a result of a steel-to-steel rolling contact.